Fatigue damage in bone due to repeated mechanical loading has been implicated in the development of degenerative disorders, avascular necrosis, osteochondritis, senile femoral neck fractures, spondylolisthesis, pathological fractures and fatigue fractures. A comprehensive study of bone fatigue damage and the accompanying repair processes will yield basic information on bone physiology leading to improved patient management. The proposed investigation will consist of two closely coordinated research programs: 1) in vitro studies; 2) in vivo canine studies. The in vitro studies will be used to determine the mechanical behavior of human cortical bone specimens subjected to repeated loading at physiologic loading rates. The differences between tensile, compressive, and fully reversed fatigue loading will be examined. The influence of bone density, ash weight, and microstructure will be determined. The loss of bone strength and stiffness during repeated loading will be studied and microscopic examinations of fatigue damage will be conducted. The in vivo studies will examine bone fatigue damage and repair in adult mongrel dogs. Strain gage rosettes bonded to one radius anteriorly will be used to estimate the loading patterns in the radii before and after bilateral ulna resection. A controlled exercise program will then impose specific loading histories on the experimental radii. Tetracycline labeling will be used to document the extent of bone repair and remodeling in the contralateral radius. The remodelling response will also be following using radioisotope bone scans and roentgenograms.